JP2001-082302A published by the Japanese Patent Office in 2001 discloses ignition timing control of an internal combustion engine using a rotation speed of the engine as a parameter. A crank angle sensor detects the engine rotation speed. The crank angle sensor outputs a signal when the crankshaft of the engine reaches a defined reference rotation position for each cylinder.
A separate signal is outputted when the crank shaft rotates through a unit angle which is set for example to one degree. The former signal is termed a reference position signal or a REF signal and the latter is termed a unit crank angle signal or a POS signal.
The engine rotation speed is obtained by measuring the interval between the REF signal and the POS signal. Since the POS signal is updated more frequently than the REF signal, the rotation speed obtained from the POS signal has a higher tracking ability of the real rotation speed of the engine than that obtained from the REF signal.